1. Field of the Invention
The present invention generally relates to a heat exchanger, such as a condenser or an evaporator, and more particularly, to heat exchangers including heat exchange units at which an exchange of heat occurs, that have openings and louvers.
2. Description of the Prior Art
A heat exchanger, such as an evaporator for use in an automotive air conditioning systems, as illustrated in FIG. 1, is well known in the art. For example, such heat exchangers are described in Japanese Patent Application Publication No. 6-117790, which is incorporated herein by reference.
Referring to FIG. 1, an evaporator 300 includes an upper tank 310 and a lower tank 320 which is vertically spaced from upper tank 310. Upper and lower tanks 310 and 320 are made of an aluminum alloy and are rectangular parallelepiped in shape. Moreover, each of tanks 310 and 320 has a length l.sub.t and a width w.sub.t. Evaporator 300 further includes a plurality of hem exchange units 330 at which an exchange of heat occurs. Each of heat exchange units 330 also may be made of an aluminum alloy and includes a plurality of circular pipe portions 331 and a plurality of plane portions 332 which connect adjacent pipe portions 331. The intervals between pipe portions 331 are about equal.
Heat exchange units 330 are arranged in parallel along length l.sub.t of tanks 310 and 320 at about equal intervals and extend between upper and lower tanks 310 and 320. Upper and lower tanks 310 and 320 are placed in fluid communication through pipe portions 331. Pipe portions 331 of adjacent heat exchange units 330 are offset by one half of the length of the interval between pipe portions 331 of heat exchange unit 330. The length of heat exchange units 330 is designed to be substantially equal to the width w.sub.t of tanks 310 and 320, and heat exchange units 330 have longitudinal axes parallel to the width w.sub.t of tanks 310 and 320. Pipe portions 331 and plane portions 332 may be formed integrally from an aluminum alloy plate (not shown), for example, by extrusion. As shown in FIG. 4, the thickness t.sub.pipe of the walls of pipe portions 331 is designed to be greater than the thickness t.sub.plane of plane portions 332, so that pipe portions 331 are reinforced to sufficiently resist the internal pressure.
Referring to FIGS. 3-6, considered in view of FIG. 1, evaporator 300 is provided with a plurality of diagonally arranged first louvers 333 and a plurality of diagonally arranged second louvers 334 formed in plane portions 332 of heat exchange units 330. A method of forming first and second louvers 333 and 334 is described as follows. As shown in FIG. 2, a plurality of slant slits 335 are slit in each of plane portions 332 of heat exchange unit 330 generally along the longitudinal axis of heat exchange unit 330, for example, by press work. Slits 335 are spaced at about equal intervals W.sub.s. Accordingly, a plurality of identical plane belt regions 336 are defined between adjacent slits 335. Plane belt regions 336 are alternately bulged in opposite directions from plane portion 332, for example, by press work. The above slitting and bulging steps may be accomplished, for example, by a single press work operation.
As a result of the bulging of plane belt regions 336, plane belt regions 336 are formed into first and second louvers 333 and 334, respectively, as illustrated in FIGS. 3-6. First and second louvers 333 and 334 alternately follow one another. Each of first louvers 333 includes a flat roof section 333a and a pair of inclined leg sections 333b which connect roof section 333a to plane portion 332. Flat roof section 333a is parallel to plane portion 332 and is generally rhomboidal in shape. Thus, referring to FIG. 4, pairs of windows 333c having a generally trapezoidal configuration are formed at each upper and lower edge of first louvers 333, respectively.
Similarly, each of second louvers 334 includes a flat roof section 334a and a pair of inclined leg sections 334b which connect roof section 334a to plane portion 332. Flat roof section 334a is parallel to plane portion 332 and also is generally rhomboidal in shape. Thus, pairs of windows 334c having a generally trapezoidal configuration are formed at each upper and lower edge of second louvers 334, respectively. By providing first and second louvers 333 and 334, plane portions 332 function as fin members. Further, although only some of first and second louvers 333 and 334 located at upper and lower end portions of the end heat exchange unit 330' are depicted in FIG. 1, first and second louvers 333 and 334 are formed on the entire surface of each of plane portions 332 of each of heat exchange units 330.
Referring again to FIG. 1, the interior space of upper tank 310 is divided by a partition plate 340 into a first chamber section 310a and a second chamber section 310b. Upper tank 310 is provided with an inlet pipe 350 fixedly connected through an outside end surface of section 310a and an outlet pipe 360 fixedly connected through an outside end surface of section 310b.
Further, when evaporator 300 is installed, heat exchange units 330 are oriented, so that plane portions 332 are parallel to the flow direction "A" of air passing through evaporator 300, as illustrated in FIG. 1. Consequently, pipe portions 331 are perpendicular to the flow direction "A" of air passing through evaporator 300, as illustrated in FIGS. 3, 4, and 6.
During operation of the automotive air conditioning system, the refrigerant fluid is conducted into first chamber section 310a of the upper tank 310 from an element of the automotive air conditioning system, such as a condenser (not shown), via inlet pipe 350. The refrigerant fluid in the first chamber section 310a of upper tank 310 then flows downwardly through each of pipe portions 331 of a first group of heat exchange units 330. As the refrigerant fluid flows downwardly through each of pipe portions 331 of this first group of heat exchange units 330, the refrigerant exchanges heat with the air flowing across exterior surfaces of heat exchange units 330, so that heat from the air is absorbed through plane portions 332.
The refrigerant fluid flowing downward through pipe portions 331 of this first group of heat exchange units 330 flows into a first portion of an interior space of lower tank 320, which corresponds to section 310a. Thereafter, the refrigerant fluid in the first portion of the interior space of lower tank 320 flows towards a second portion of the interior space of lower tank 320, which corresponds to section 310b. The refrigerant then flows upward from the second portion of the interior space of lower tank 320 through each of pipe portions 331 of a second group of heat exchange units 330. As the refrigerant fluid flows upwardly through each of pipe portions 331 of the second group of heat exchange units 330, the refrigerant further exchanges heat with the air flowing across the exterior surfaces of heat exchange units 330, so that the heat from the air is further absorbed through plane portions 332.
The refrigerant fluid flowing upward through each of pipe portions 331 of the second group of heat exchange units 330 flows into second chamber section 310b of upper tank 310. The refrigerant fluid in second chamber section 310b of upper tank 310 then is conducted to other elements of the automotive air conditioning system, such as a compressor (not shown), via outlet pipe 360.
However, in heat exchangers, such as those described above, performance of heat exchanger, e.g., evaporator 300, is generally insufficient. As shown in FIG. 6, air passing through evaporator 300 is cut by the upper edge of first louvers 333 (or second louvers 334). These edges have an effective length l defined by equation (1) as follows: EQU l=L.sub.L .multidot.sin .theta. (1)
In equation (1), L.sub.L is the actual length of the upper edge of first louvers 333 (or second louvers 334), and theta .theta. is an angle created between the upper edge of first louvers 333 (or second louvers 334) and the flow direction "A" of air passing through heat exchanger 300. Further, the length L.sub.L of the upper edge of first louvers 333 (or second louvers 334) is approximately equal to the length L.sub.s of slits 335. Front edge effect is the increase in heat transmission from air to a louver by cutting the air flow by a front, i.e., leading, edge of the louver. In addition, for purposes of simplicity of explanation, only first louvers 333 are described hereinafter because the functioning of second louvers 334 is substantially the same as that of first louvers 333.
According to equation (1), when the degrees of angle theta .theta. increase in a range between 0.degree. and +90.degree., the effective length l increases. Thus, with respect to first louvers 333, the following relationships are observed:
a. Angle Theta .theta..varies.Effective Length l; PA1 b. Effective Length l .varies.Front Edge Effect; PA1 c. Front Edge Effect .varies.Heat Transfer Rate; and PA1 d. Heat Transfer Rate .varies.Performance of Evaporator. PA1 a. Angle Theta .theta..varies.Length L.sub.L ; PA1 b. 1/(Length L.sub.L).varies.Fin Efficiency; and PA1 c. Fin Efficiency .varies. Performance of Evaporator.
Accordingly, if the interval between adjacent pipe portions 331 of heat exchange unit 330 is fixed, the performance of evaporator 300 is directly proportional to angle theta .theta.. Thus, when the degrees of angle theta .theta. increase, the heat transfer rate, i.e., the heat transfer coefficient, of first louvers 333 increases, so that the performance of evaporator 300 also increases.
On the other hand, when the interval between adjacent pipe portions 331 of heat exchange unit 330 is fixed, when the degrees of angle theta .theta. increase, the length of first louvers 333 increases. Further, the length L.sub.L of first louvers 333 is also approximately equal to the length L.sub.s of slits 335. Thus, with respect to first louvers 333, the following relationships are observed:
Accordingly, if the interval between the adjacent pipe portions 331 of heat exchange unit 330 is fixed, the performance of evaporator 300 is inversely proportional to angle theta .theta.. Thus, when the degrees of angle theta .theta. increase, the fin efficiency of first louvers 333 decreases, so that the performance of evaporator 300 also decreases.
As described above, the heat transfer rate and the fin efficiency of first louvers 333 are functions of angle theta .theta., but changes in angle theta .theta. have opposite effects on heat transfer rate and fin efficiency, which in turn cause opposite effects on performance of evaporator 300. Accordingly, in the heat exchangers discussed above, the performance is insufficient. Therefore, it is desirable to set angle theta .theta. at a certain value at which the contributions of the heat transfer rate and the fin efficiency of louvers 333 to the performance of evaporator 300 are balanced.